Continuous strand mats have been glass fiber reinforcements for plastics for many years. The mats have strands of infinite length in a random orientation which look like lazy whirl formations with each strand assuming an individualistic pattern or overlapping as coils. The industry gathers the strands on a conveyor, bonds them with a binder, cures and rolls them as flat goods to be shipped to a molder. Laminated moldings with continuous strand mat as reinforcements have isotropic mechanical properties. That is, mechanical properties such as tensile strength, flexural strength and impact strength that are generally identical in all directions. Other stranded glass fiber reinforcements that can be rolled up as flat goods and shipped to molders exist. Examples are chopped strand mat, woven roving, woven glass fiber fabrics, braided strands, and knitted fabrics which are unidirectional, bidirectional or multidirectional. One distinction continuous strand mat has over the other flat goods is that continuous strand mat can be stretched during molding to form complex contoured shapes.
Continuous strand mats have been reinforcements for several molding processes such as matched compression molding, pultrusion, Resin Transfer Molding (RTM) and Structural Reaction Injection Molding (SRIM).
The industry currently is showing renewed interest in the RTM and SRIM processes as efficient methods to produce large complicated shapes for use in the automotive industry. One important feature of the RTM and SRIM processes is that of parts consolidation. Certain automotive parts that previously required one or more steel stampings welded together to make a single part, now can be made as a single part in one operation by using the RTM or SRIM processes. The RTM and SRIM processes have an additional feature of extreme importance. The processes can include other materials such as rigid foam, steel support plates, wiring, and tubing incorported during the molding process.
One particular example of a large complicated automotive structure is a crossmember structure. This part, which currently has ten steel stampings, can be made as one molding by using the RTM process with glass fiber reinforcements and resins.
Layers of continuous strand mat can form the required shape (preforms). Other materials such as directional reinforcements and blocks of rigid foam can be added to the preform. The preforms and additions are placed in a mold and injected with a catalyzed resin (RTM) to make the crossmember structure.
This large a structure requires added directional reinforcements placed on the bias to this part so that required torsional rigidity may be imparted to the structure. One problem encountered in using directional reinforcements to make a preform is that directional fibers do not have the ability to stretch so as to conform to complex compound contours. What is needed is a directional reinforcement that stretches to conform to complex compound contours.